Snakelike Skin Gives a Robot the Power to Crawl

Snakes, serpents, danger noodles. Whatever you call them, you’ve gotta respect them. I mean, have you tried getting around without any arms or legs? (Also, they can bite you.)

The snake’s ambulatory secret is its special belly scales, which grip a surface like cleat spikes to help the reptile push forward. And now that secret has made it into robotics. Researchers report today in Science Robotics that they’ve designed an inflatable robot with its own scales that automatically pop out to get that grip. Even cooler still: The scientists laser-cut different shapes of scales, from circles to triangles to trapezoids, and it turns out that not all shapes are created equal.

The skin’s intricate structure, inspired by the Japanese art of kirigami.

Ahmad Rafsanjani/ Bertoldi Group/ Harvard University

The system is elegant in its simplicity. The core of the robot is an inflatable tube—one that inflates mostly lengthwise, not in girth. Over this the researchers applied a polyester skin snipped in the style of kirigami, or the Japanese tradition of paper cutting. Essentially, they created lots of tiny flaps.

“This pattern, when you stretch it, some parts go up and some parts go down,” says Harvard mechanical engineer Ahmad Rafsanjani, lead author on the paper. “So we came up with some new designs, which is basically unidirectional kirigami features. They all go in one direction.”

By pumping air into the tube, the whole structure extends forward, while the scales pop out, digging into the ground like ice picks. The resulting friction allows the robot to essentially drag itself forward, inching bit by bit as it inflates and deflates.

It’s a clever development for the newish field of soft robotics, which typically use oil or air to move—as opposed to traditional robots, which use electric motors called actuators. Think things like this robot muscle that pumps oil to lift objects. Or these origami-inspired soft robots that come complete with skeletons. Without actuators, it can be hard to make a soft robot do anything but flex in place, but this development shows a way forward.

Curiously, the efficiency of the bot’s movement depends on the shape of the scales the researchers cut. Trapezoidal scales pull the snake farther with each inflation than circular scales, which are better than triangular. Why exactly this is, Rafsanjani can’t yet say for sure. But it may have something to do with extra surface area a trapezoid provides over circles or triangles.

“If you look at the belly scales of snakes, they look more trapezoidal,” says Rafsanjani. “It comes to mind that if you have more surface area, there’s more contact. So if you have a very rough surface, there’s more chance to grab.”

What’s interesting here is that the researchers didn’t set out to exactly mimic a snake. They experimented with different shapes and just so happened to show that a snake’s belly scales may be pretty darn optimized for creating friction. And theoretically these scientists can keep iterating to create a system that pulls off maneuvers snakes never could. We see this with another snake robot from Carnegie Mellon, which moves like a snake but can also move in ways biology doesn’t allow.

At the moment, this new system isn’t the speediest way for a robot to get around. But maybe these robots’ descendants won’t need to be all that speedy. They might be able to squeeze into tight places, given they’re pretty much only expanding lengthwise, making them good for things like inspections. Or if they get small enough, maybe they could crawl through your arteries one day. That’s right, robot snakes in your arteries.